Image forming apparatus and curl detecting method

ABSTRACT

An image forming apparatus includes a first sensor lever which rotates between a standby position where the first sensor lever crosses a sheet conveyance path obliquely from a first rotation shaft to a sheet conveyance direction downstream side of the first rotation shaft and a first sensor is turned one of ON and OFF and a retract position where the first sensor lever is retracted from the sheet conveyance path and the first sensor is turned the other of ON and OFF, and a second sensor lever which rotates between a standby position where the second sensor lever crosses the sheet conveyance path obliquely from a second rotation shaft to the sheet conveyance direction downstream side of the second rotation shaft and a second sensor is turned one of ON and OFF and a retract position where the second sensor lever is retracted from the sheet conveyance path and the second sensor is turned the other of ON and OFF.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is based upon and claims the benefit of priority from: U.S. provisional application 61/182,222, filed on May 29, 2009, the entire contents of which are incorporated herein by reference.

FIELD

The specification relates to curl detection of a sheet-like recording medium as an object of an image forming process in an image forming apparatus.

BACKGROUND

Hitherto, in an image forming apparatus, there is known a technique to detect whether a sheet as a recording medium is curled.

In the image forming apparatus of the related art, a process to prevent degradation of image formation quality, such as curl correction of the sheet using a curl correcting mechanism (decurler), is performed based on the detection result of curl of the sheet.

Specifically, curl detecting techniques as in the following (1) and (2) are known.

(1) A technique in which the variation amount of a movable guide varying in accordance with the curl of the sheet is detected to detect the presence or absence of curl of the sheet.

(2) A technique in which two sensors are arranged along a sheet conveyance direction at the sheet conveyance direction downstream side of a heat fixing unit, and detects the presence or absence of curl of the sheet

However, in the curl detecting techniques of the related art, there are problems that the structure for detecting the curl of the sheet is complicated, or it is difficult to grasp which direction the sheet is curled in.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an image forming apparatus.

FIG. 2 is a longitudinal sectional view of the periphery of a curl detecting device D.

FIG. 3 is a schematic perspective view of the periphery of the curl detecting device D in the state of FIG. 2.

FIG. 4 is a view for explaining the details of a structure of the curl detecting device D.

FIG. 5 is a longitudinal sectional view showing a state where a non-curled sheet passes through the curl detecting device D.

FIG. 6 is a schematic perspective view of the periphery of the curl detecting device D in the state of FIG. 5.

FIG. 7 is a longitudinal sectional view showing a state just before the sheet comes out of the curl detecting device D.

FIG. 8 is a schematic longitudinal sectional view showing a relation between a sensor lever and an inner wall of a sheet conveyance path.

FIG. 9 is a first operation explanatory view of the curl detecting device D.

FIG. 10 is a second operation explanatory view of the curl detecting device D.

FIG. 11 is a third operation explanatory view of the curl detecting device D.

FIG. 12 is a fourth operation explanatory view of the curl detecting device D.

FIG. 13 is a fifth operation explanatory view of the curl detecting device D.

FIG. 14 is a sixth operation explanatory view of the curl detecting device D.

FIG. 15 is a seventh operation explanatory view of the curl detecting device D.

FIG. 16 is an eighth operation explanatory view of the curl detecting device D.

FIG. 17 is a ninth operation explanatory view of the curl detecting device D.

FIG. 18 is a longitudinal sectional view showing a schematic structure of a curl detecting device of a second embodiment.

DETAILED DESCRIPTION

In general, according to an embodiment, an image forming apparatus includes a sheet conveyance path, a first sensor, a first sensor lever, a first urging section, a second sensor, a second sensor lever and a second urging section.

The sheet conveyance path guides a sheet in a sheet conveyance direction. The first sensor lever rotates around a first rotation shaft which is substantially parallel to a surface perpendicular to the sheet conveyance direction and is substantially parallel to a surface of the sheet conveyed in the sheet conveyance path, and rotates between a standby position where the first sensor lever crosses the sheet conveyance path obliquely from the first rotation shaft to a sheet conveyance direction downstream side of the first rotation shaft and the first sensor is turned one of ON and OFF and a retract position where the first sensor lever is retracted from the sheet conveyance path and the first sensor is turned the other of ON and OFF. Besides, the first urging section urges the first sensor lever from the retract position side to the standby position side. The second sensor lever rotates around a second rotation shaft which is substantially parallel to the surface perpendicular to the sheet conveyance direction and is substantially parallel to the surface of the sheet conveyed in the sheet conveyance path, and rotates between a standby position where the second sensor lever crosses the sheet conveyance path obliquely from the second rotation shaft to the sheet conveyance direction downstream side of the second rotation shaft and the second sensor is turned one of ON and OFF and a retract position where the second sensor lever is retracted from the sheet conveyance path and the second sensor is turned the other of ON and OFF. Besides, the second urging section urges the second sensor lever from the retract position side to the standby position side.

First Embodiment

Hereinafter, an embodiment will be described with reference to the drawings.

FIG. 1 is a longitudinal sectional view of an image forming apparatus.

The image forming apparatus includes an image reading section R and an image forming section P.

The image reading section R scans and reads an image of a sheet document and a book document.

The image forming section P forms a developer image on a sheet based on the image read from the document by the image reading section R or image data transmitted to the image forming apparatus from an external equipment.

The image reading section R includes an auto document feeder 9. The image reading section R reads the image of the document placed on a document tray Rt and automatically fed by the auto document feeder 9 or the document placed on a document table J.

The image forming section P includes pickup rollers 51 to 54, photoreceptors 2Y to 2K, development rollers 3Y to 3K, mixers 4Y to 4K, cleaning units 6Y to 6K, an intermediate transfer belt 11, a secondary transfer roller 12, a fixing device 7, a discharge tray 8, toner cartridges 1Y to 1K, a laser sweep unit L, a curl detecting device D and an ADU (Auto Duplex Unit) 15.

The image forming apparatus includes a processor 801, an ASIC (Application Specific Integrated Circuit) 802, a memory 803, a HDD (Hard Disk Drive) 804, an operation input section 805 and a display 806. The processor 801 performs various processes in the image forming apparatus. The processor 801 executes programs stored in the memory 803 or the HDD 804 and realizes various functions.

It is needless to say that the processor 801 can be realized by a CPU (Central Processing Unit) or an MPU (Micro Processing Unit) which can execute an equivalent arithmetic processing. Besides, similarly, the HDD 804 can be substituted by a storage device such as a flash memory.

The memory 803 may be, for example, a RAM (Random Access Memory), a ROM (Read Only Memory), a DRAM (Dynamic Random Access Memory), an SRAM (Static Random Access Memory), a VRAM (Video RAM), a flash memory or the like.

The operation input section 805 can be composed of, for example, a keyboard, a mouse, a touch panel, a touch pad, a graphics tablet, a dedicated button or the like.

The display 806 can be composed of, for example, an electric paper, an LCD (Liquid Crystal Display), an EL (Electronic Luminescence), a PDP (Plasma Display Panel), a CRT (Cathode Ray Tube) or the like.

The functions of the operation input section 805 and the display 806 can also be realized by a so-called touch panel display.

Hereinafter, as an example of a process in the image forming apparatus, the outline of a copy process will be described.

First, the pickup rollers 51 to 54 convey sheets from cassettes to a sheet conveyance path. Plural roller pairs convey the sheets to a secondary transfer position T.

The image reading section R reads images of plural sheet documents to obtain image data.

The laser sweep unit L forms electrostatic latent images on photoconductive surfaces of the photoreceptors 2Y, 2M, 2C and 2K based on the image data.

The mixers 4Y to 4K in developing units agitate developer in the developing units. The development rollers 3Y to 3K supply the developer to the electrostatic latent images on the photoreceptors 2Y to 2K, and visualize the electrostatic latent images on the photoconductive surfaces of the photoreceptors. The toner cartridges 1Y to 1K supply toner to the developing units.

The photoreceptor transfers the developer image onto a belt surface of the intermediate transfer belt 11 (so-called primary transfer). The intermediate transfer belt 11 conveys the developer image carried on the rotating belt surface to the secondary transfer position T. The intermediate transfer belt 11 and the secondary transfer roller 12 nip and convey the sheet at the secondary transfer position T, and transfer the toner image on the intermediate transfer belt 11 to the sheet.

The fixing device 7 heats and fixes the developer image on the sheet to the sheet.

The curl detecting device D detects the direction of a curl of the sheet passing through the fixing device 7 and the degree of the curl.

The ADU 15 uses an inversion conveyance path 15 p to invert the front and back of the sheet subjected to the curl detection by the curl detecting device D when necessary, and feeds it to the fixing device 7 again.

The plural conveyance roller pairs successively discharge the sheet after heating and fixing of the developer image onto the discharge tray 8.

Hereinafter, the details of the structure of the curl detecting device D will be described.

FIG. 2 is a longitudinal sectional view of the periphery of the curl detecting device D just before a non-curled sheet enters the device. FIG. 3 is a schematic perspective view of the periphery of the curl detecting device D in the state of FIG. 2. FIG. 4 is a view for explaining the details of the structure of the curl detecting device D. FIG. 5 is a longitudinal sectional view showing a state where a non-curled sheet passes through the curl detecting device D. FIG. 6 is a schematic perspective view of the periphery of the curl detecting device D in the state of FIG. 5. FIG. 7 is a longitudinal sectional view showing the state just before the sheet comes out of the curl detecting device D. FIG. 8 is a schematic longitudinal sectional view showing a relation between a sensor lever constituting the curl detecting device D and an inner wall of the sheet conveyance path. FIG. 9 to FIG. 17 are schematic views showing the behavior of the curl detecting device D during the period after a curled sheet enters the curl detecting device D and before the sheet comes out of it.

As shown in FIG. 2, the curl detecting device D includes, for example, a first sensor lever 110, a first rotation shaft 110 a, a first urging section 113 (see FIG. 4), a first sensor S1, a support section K1, a conveyance guide g1, a second sensor lever 120, a second rotation shaft 120 a, a second urging section 123 (see FIG. 4), a second sensor S2, a support section K2 and a conveyance guide g2.

The sheet conveyance path includes the conveyance guide g1 and the conveyance guide g2. The sheet conveyance path guides a sheet conveyed through a sheet passing space in the sheet conveyance path to a sheet conveyance direction (x-axis direction). Here, the size of the sheet passing space in a direction perpendicular to the surface of the sheet conveyed in the sheet conveyance path is called “gap” (see FIG. 4).

The first sensor S1 is a transmission type optical sensor.

The first sensor lever 110 rotates around the first rotation shaft 110 a (shaft extending in the y-axis direction) which is substantially parallel to the surface (z-y plane) perpendicular to the sheet conveyance direction and is substantially parallel to the surface (x-y plane) of the sheet conveyed in the sheet conveyance path.

The first sensor lever 110 rotates between a standby position where the first sensor lever (at least a sheet contact surface (see FIG. 2)) obliquely crosses the sheet conveyance path from the first rotation shaft 110 a to the sheet conveyance direction downstream side of the first rotation shaft 110 a and the first sensor S1 is turned one of ON and OFF and a retract position where the first sensor lever is retracted from the sheet conveyance path and the first sensor S1 is turned the other of ON and OFF.

The first urging section 113 urges the first sensor lever 110 from the retract position side to the standby position side (counterclockwise direction around the first rotation shaft 110 a in FIG. 4).

The second sensor S2 is a transmission type optical sensor.

The second sensor lever 120 rotates around the second rotation shaft 120 a (shaft extending in the y-axis direction) which is substantially parallel to the surface (z-y plane) perpendicular to the sheet conveyance direction and is substantially parallel to the surface (x-y plane) of the sheet conveyed in the sheet conveyance path.

The second sensor lever 120 rotates between a standby position where the second sensor lever (at least a sheet contact surface (see FIG. 2)) obliquely crosses the sheet conveyance path from the second rotation shaft 120 a to the sheet conveyance direction downstream side of the second rotation shaft 120 a and the second sensor S2 is turned one of ON and OFF and a retract position where the second sensor lever is retracted from the sheet conveyance path and the second sensor S2 is turned the other of ON and OFF.

The second urging section 123 urges the second sensor lever 120 from the retract position side to the standby position side (clockwise direction around the second rotation shaft 120 a in FIG. 4).

The first sensor lever 110, the first sensor S1, the first urging section 113, the support section K1 and the conveyance guide g1 are axisymmetric (mirror image relation with respect to a sheet conveying locus) with the second sensor lever 120, the second sensor S2, the second urging section 123, the support section K2 and the conveyance guide g2 with respect to the sheet conveying locus in the sheet conveyance direction (see FIG. 4).

The conveyance guide g1 supports the first sensor lever 110, the first sensor S1, the first rotation shaft 110 a and the support section K1. The conveyance guide g2 supports the second sensor lever 120, the second sensor S2, the second rotation shaft 120 a and the support section K2.

The first sensor lever 110 can cross the sheet conveyance path through a through hole h1 passing through from the outside of the conveyance guide g1 to the outside of the conveyance guide g2. The second sensor lever 120 can cross the sheet conveyance path through a through hole h2 passing through from the outside of the conveyance guide g2 to the outside of the conveyance guide g1.

The first sensor lever 110 includes a lever section 111 and a blocking section 112 while the first rotation shaft 110 a is the center. The second sensor lever 120 includes a lever section 121 and a blocking section 122 while the second rotation shaft 120 a is the center.

The first sensor lever 110 and the second sensor lever 120 block the detection light of the first sensor S1 and the second sensor S2 at the standby position and bring the first sensor S1 and the second sensor S2 into an OFF state (see FIG. 2 and FIG. 3). When a sheet P enters the curl detecting device D and the first sensor lever 110 and the second sensor lever 120 are pressed by the leading edge of the sheet P and are rotated by a specified angle or more, the detection light of the first sensor S1 and the second sensor S2 comes not to be blocked, and the first sensor S1 and the second sensor S2 are brought into an ON state (see FIG. 5 and FIG. 6).

In the conveyance guides g1 and g2 constituting the sheet conveyance path, when viewed in the first rotation shaft 110 a direction (y-axis direction in FIG. 4), the gap in a range V from a crossing position C1 of the sheet contact surface (see FIG. 2) of the first sensor lever 110 in the sheet conveyance direction and the sheet contact surface of the second sensor lever 120 to the first rotation shaft 110 a is wider than the gap at a position E adjacent to the sheet conveyance direction upstream side of the first rotation shaft 110 a in the sheet conveyance direction (see reference sign bf1 in FIG. 4).

Besides, an inner wall surface of the expanding portion bf1 in the vicinity of the crossing position C1 of the sheet conveyance path has a shape which gradually expands from the sheet conveyance direction upstream side and then gradually narrows.

The space where the leading edge of the curled sheet can enter is provided in the vicinity of the sheet conveyance direction upstream side of the sheet contact surface of each of the sensor levers at the standby position. Thus, a time elapsed before the leading edge of the curled sheet contacts the sheet contact surface of the sensor lever can be made long, and consequently, a time difference between the detection timing of the first sensor S1 and the detection timing of the second sensor S2 can be made longer.

The sheet contact surface (see FIG. 2) of the first sensor lever 110 and the second sensor lever 120 at the standby position enters the sheet conveyance path from the inner wall surface of the gradually expanding portion in the vicinity of the crossing position C1 (see reference sign Q in FIG. 4).

Besides, each of the sheet contact surface (see FIG. 2) of the first sensor lever 110 and the sheet contact surface of the second sensor lever 120 has an inclination of θ (=30 degrees or more) with respect to the sheet conveying locus at the standby position. The sheet contact surface of each of the sensor levers is significantly inclined with respect to the sheet conveying locus, so that, when the curl of the sheet is generated, the time difference between the detection timing of the first sensor S1 and the detection timing of the second sensor S2 can be made longer. That is, the sheet contact surface of each of the sensor levers is significantly inclined with respect to the sheet conveying locus, so that the accuracy of the curl detection can be raised.

Besides, with respect to the conveyance guides g1 and g2 constituting the sheet conveyance path, when viewed in the first rotation shaft direction (y-axis direction in FIG. 4), the gap at a leading edge position 111 t of the first sensor lever 110 and a leading edge position 121 t of the second sensor lever 120 (see reference sign bf2 in FIG. 4) at the standby position in the sheet conveyance direction is wider than the gap in the position E adjacent to the sheet conveyance direction upstream side of the first rotation shaft 110 a in the sheet conveyance direction.

Besides, with respect to the conveyance guides g1 and g2 constituting the sheet conveyance path, when viewed in the first rotation shaft direction (y-axis direction in FIG. 4), the gap at sheet conveyance direction downstream side end positions of the sheet contact surfaces (see FIG. 2) of the first sensor lever 110 and the second sensor lever 120 at the retract position is wider than the gap in the position E adjacent to the sheet conveyance direction upstream side of the first rotation shaft 110 a in the sheet conveyance direction (See FIG. 5).

Besides, with respect to the conveyance guides g1 and g2 constituting the sheet conveyance path, when viewed in the first rotation shaft direction (y-axis direction in FIG. 4), the gap at the position of the first rotation shaft 110 a in the sheet conveyance direction is wider than the gap in the position E adjacent to the sheet conveyance direction upstream side of the first rotation shaft 110 a in the sheet conveyance direction.

Next, an operation of the curl detecting device D will be described.

In the image forming apparatus shown in FIG. 1, it is assumed that a curl in which a recording surface side of a sheet becomes a concave surface is such that an upper surface side in FIG. 7 becomes a convex surface (the sheet leading edge is directed downward), and a curl in which the recording surface side of the sheet becomes a convex surface is such that a lower surface side in FIG. 7 becomes a convex surface (the sheet leading edge is directed upward).

In the state of FIG. 7, the blocking section 112 of the first sensor lever 110 is positioned at a changing point where ON changes to OFF with respect to the optical path of the first sensor S1. Similarly, in the state of FIG. 7, the blocking section 122 of the second sensor lever 120 is positioned at a changing point where ON changes to OFF with respect to the optical path of the second sensor S2.

When the sheet is further conveyed left (sheet conveyance direction) from the state of FIG. 7, both the first sensor S1 and the second sensor S2 are placed in the stable state of OFF.

In FIG. 7, it is assumed that the sheet is conveyed in the direction from right to left, and a distance between an ON point and an OFF point of the first sensor S1 and the second sensor S2 when the sheet hardly curls is L0. In the example shown in FIG. 7, L0 is approximately 14 mm.

Besides, a timing when the first sensor S1 is turned ON is αon, a timing when the first sensor S1 is turned OFF is αoff, a timing when the second sensor S2 is turned ON is βon, and a timing when the second sensor S2 is turned OFF is βoff.

As an example, a description will be made to a detection state in the first sensor S1 and the second sensor S2 when a relatively large curl state where a upper side becomes a convex surface occurs at the time point when a fixing process of a toner image to the sheet is ended.

In the curl state where the upper side becomes the convex surface (sheet leading edge is directed downward), the leading edge detection of the sheet becomes such that βon first occurs, and next, αon occurs.

Besides, in the trailing edge detection of the sheet in the curl state where the upper surface side becomes the convex surface (sheet trailing edge is directed downward), αoff first occurs, and next, βoff occurs.

That is, in the curl state where the upper surface side becomes the convex surface, the sheet leading edge and the trailing edge pass through the sheet conveyance path in the state where the sheet contacts the inner wall surface of the conveyance guide g2 or the state where the sheet is positioned in the vicinity of the inner wall surface of the conveyance guide g2. Accordingly, in the second sensor S2 positioned at the conveyance guide g2 side, as compared with the first sensor S1 positioned at the conveyance guide g1 side, the time of the ON state is long.

FIG. 8 shows a case where a sheet in the curl state where the upper surface side becomes the convex surface passes through the curl detecting device D, and the distance between the ON point and the OFF point of the second sensor S2 becomes maximum. In FIG. 8, for convenience of explanation, the sensor lever 110 is omitted.

In FIG. 8, the sheet contact position where the sensor is turned ON is shifted to the right of the position shown in FIG. 7, and the sheet contact position where the sensor is turned OFF is shifted to the left of the position shown in FIG. 7. The longest distance Lmax from the sheet contact position where the sensor is turned ON to the sheet contact position where the sensor is turned OFF is about 30 mm. For convenience of explanation, FIG. 8 shows also the shortest distance Lmin (=about 11 mm).

As shown in FIG. 8, when the sheet is relatively highly curled, the distance from ON position of each sensor to OFF position varies, and varies in a range of about 11 mm to about 30 mm.

As an example, a description will be made to a case where the conveyance speed of the sheet is 500 [mm/sec], and the short side (197 mm) of A4 sheet passes through the curl detecting device D.

In this case, the time when the ON state of the sensor continues varies in the range of about 0.414 to about 0.454 [sec] according to the state of the curl as compared with the time of

(197+14)/500=0.422 [sec]

where there is no curl.

Specifically, when the sheet in the curl state where the lower surface side becomes the convex surface passes through the curl detecting device D, the detection results of the first sensor S1 and the second sensor S2 are as follows:

αon→βon→βoff→αoff

Besides, when the sheet in the curl state where the upper surface side becomes the convex surface passes through the curl detecting device D, the detection results of the first sensor S1 and the second sensor S2 become as follows:

βon→αon→αoff→βoff

FIG. 10 shows a state of βon, FIG. 11 shows a state of αon, FIG. 14 shows a state of αoff, and FIG. 17 shows a state of βoff

The processor 801 (curl determination section) determines that in the first sensor S1 and the second sensor S2, the sheet is curled to the side of the sensor in which the change of the detection result from one of ON and OFF to the other is earlier, and the change of the detection result from the other to the one is slower.

Besides, the processor 801 (curl determination section) determines that the curl amount of the conveyed sheet is large when the time difference between the change timing of the detection result from one of ON and OFF of the first sensor S1 to the other and the change timing of the detection result from one of ON and OFF of the second sensor S2 to the other is large.

In principle, by the difference between the detection timing of the first sensor S1 and the detection timing of the second sensor S2, it is possible to estimate the curl direction indicating which of the upper surface side and the lower surface side of the sheet is curled to become the convex surface, and the curl degree indicating the degree to which the sheet is curled.

The processor 801 determines that the sheet is curled when the time difference between the detection timing of the first sensor S1 and the detection timing of the second sensor S2 satisfies a specific condition, for example, when the time difference exceeds a specified time. The specified time can be determined based on a time for the case where for example, a sheet is detected, which is significantly curled to such a degree that some disadvantage occurs when the sheet passes through the curl detecting device D and is conveyed to a latter unit.

Based on the result determined as described above, the processor 801 executes curl countermeasure processes as described below.

(1) Alarm Display for Preventing Conveyance Jam

The processor 801 causes the display 806 to display that when the significantly curled sheet is conveyed to the sheet conveyance direction downstream side of the curl detecting device D, there is a fear that a sheet jam occurs.

(2) Alarm Display for Restriction of Finisher Use

When the image forming apparatus includes a finisher capable of executing processes such as stapling, punching, book binding and sorting, the processor 801 causes the display 806 to display that since the sheet is significantly curled, there is a possibility that a problem occurs in the process.

(3) Alarm Display for Restriction of Paper Discharge Alignment

When the image forming apparatus includes a finisher capable of discharging a sheet while the sheet is aligned, the processor 801 causes the display 806 to display that since the sheet is significantly curled, there is a possibility that a problem occurs in an alignment process, or a restriction is applied to the processing content of the alignment process.

(4) Alarm Display for Confirmation of Sheet Type Setting

When the image forming apparatus can apply different fixing temperatures and the like according to the sheet type such as sheet material or thickness, the processor 801 causes the display 806 to display that since the sheet is significantly curled, it should be confirmed whether the setting of the sheet type, such as presently set sheet material and thickness, is appropriate.

(5) Condition Setting and Decurler Driving when a Decurler Function is Provided

When the image forming apparatus includes a decurler to remove the curl of a curled sheet, and when the degree of the curl of the sheet exceeds a specified value, the processor 801 executes a parameter setting process for executing a curl removal process of the decurler, and causes the decurler to execute the curl removal process. The processor 801 can relieve the curl of the sheet by the curl removal process and output the sheet.

(6) Change to Preliminary Process Condition (Low Curl Mode)

When the image forming apparatus can perform a fixing process at plural fixing temperatures according to the degree of curl of a sheet, and when the degree of the curl of the sheet significantly exceeds the specified value, the processor 801 changes the setting to a fixing temperature different from the presently set fixing temperature. By this, in the fixing process performed after it is detected that the degree of the curl of the sheet is large, the degree of the generated curl can be suppressed to be small.

Besides, at an upstream side of the expanding portion bf1 of the inner wall surface of the conveyance guide g1 and the conveyance guide g2 constituting the sheet conveyance path, an elastic sheet extending from the upstream side position onto the expanding portion bf1 may be disposed.

By disposing the elastic sheet as stated above, when the degree of the curl of the sheet conveyed to the curl detecting device D does not exceed a certain degree, the leading edge can not reach a position close to the wall surface of the expansion portion bf1 against the elastic force of the elastic sheet.

That is, when the sheet whose degree of curl is small is conveyed, since there is no sufficient time difference between the detection timing of the first sensor S1 and the detection timing of the second sensor S2, the processor 801 determines that the level of the detected curl is not such that a problem occurs.

By adopting the structure as described above, it is possible to determine that only the sheet, which is curled significantly so that an influence is exerted on a latter process, is curled.

Second Embodiment

Next, a second embodiment will be described.

The second embodiment is a modified example of the first embodiment. Hereinafter, a portion having the same function as a portion described in the first embodiment is denoted by the same reference numeral and its description is omitted.

FIG. 18 is a longitudinal sectional view showing a schematic structure of a curl detecting device of the second embodiment.

As shown in the drawing, the curl detecting device includes, for example, two transmission type photosensors, a conveyance guide g1′ and a conveyance guide g2′. The two transmission type photosensors respectively include a pair of a light emitting element PS1 and a light receiving element PR1 and a pair of a light emitting element PS2 and a light receiving element PR2.

The conveyance guide g1′ includes holes h3 and h4 through which the outside of the conveyance guide g1′ communicates with a sheet conveyance path. The conveyance guide g2′ includes a hole h5 through which the outside of the conveyance guide g2′ communicates with the sheet conveyance path.

The light emitted by the light emitting element PS1 passes through the hole h5 of the conveyance guide g2′ and the hole h3 of the conveyance guide g1′ and reaches the light receiving element PR1.

The light emitted by the light emitting element PS2 passes through the hole h5 of the conveyance guide g2′ and the hole h4 of the conveyance guide g1′ and reaches the light receiving element PR2.

Besides, the two transmission type sensors are arranged not to be axisymmetric with each other with respect to a sheet conveying locus in the sheet conveyance path composed of the conveyance guide g1′ and the conveyance guide g2′.

Specifically, an intersection point PX of the light beam emitted by the light emitting element PS1 and the light beam emitted by the light emitting element PS2 is positioned on the tangent line of the inner wall surface of the conveyance guide g2′ facing the sheet conveyance path.

Besides, the two transmission type sensors are axisymmetric with each other with respect to the line perpendicular to the sheet conveying locus passing through the intersection point PX.

Hereinafter, a description will be made to a case where a sheet in a curl state where an upper surface side is a convex surface passes through the curl detecting device of the structure shown in FIG. 18.

A detection area (sheet conveyance direction area where an optical axis is inclined) of the transmission type sensor including the light emitting element PS1 and the light receiving element PR1 is U1, and a detection area (sheet conveyance direction area where an optical axis is inclined) of the transmission type sensor including the light emitting element PS2 and the light receiving element PR2 is U2. The detection area U1 and the detection area U2 are almost continuous with each other.

For example, since the leading edge of the sheet (Pin), which is significantly curled to a certain degree so that the upper surface side becomes the convex surface, advances along the inner wall surface of the conveyance guide g2′, the leading edge of the sheet (Pin) passes through the vicinity of the intersection point PX. Accordingly, the two transmission type sensors detect (sensor ON) the leading edge of the sheet (Pin) at almost the same timing, and detect that the trailing edge of the sheet (Pout) comes out at almost the same timing.

For example, in the image forming apparatus having a tendency that a curl is generated so that an upper surface side generally becomes a convex surface by a heat fixing process, when a time difference t1 between the sensors in the detection timing of the sheet leading edge and a time difference t2 between the sensors in the detection timing of the sheet trailing edge are smaller than a specific value, the processor 801 executes a curl countermeasure process such as the curl countermeasure processes (1) to (6) described in the first embodiment.

When the conveyance speed of the sheet is 500 mm/sec, and the detection areas are U1=U2=5 mm, and when a non-curled sheet passes through an intermediate position of the conveyance guide g1′ and the conveyance guide g2′, the time differences are

t1=t2=5/500=0.01 sec=10 msec.

In the case of a large curl in which an upper surface side becomes a convex surface, the time difference t1 and the time difference t2 approach zero.

Accordingly, for example, when the condition of t1<0.004 and t2<0.004 continuously occurs for three sheets, it is determined that the present fixing process or the setting value of sheet type is in the state where a large curl in which the upper surface side becomes the convex surface is easily generated, and the curl countermeasure process such as (1) to (6) described in the first embodiment is executed.

In the foregoing respective embodiments, when the sheet as the conveyance object is conveyed in, for example, the lateral direction and is distorted by the influence of the gravity, it is necessary to determine the degree of the curl of the sheet in view of the influence of the distortion.

In this case, for example, a correction table to cancel the influence of distortion corresponding to the type of sheet is previously stored in the HDD 804, and the processor 801 may correct the estimated degree of curl by using the correction table.

The respective operations of the processes in the image forming apparatus are realized by causing the processor 801 to execute a curl detection program stored in the memory 802 or the HDD 804.

Further, a program to cause a computer of the image forming apparatus to execute the respective operations can be provided as the curl detection program. In this embodiment, although the case is exemplified in which the program for realizing the function to carry out the embodiment is previously stored in the storage area provided in the apparatus, no limitation is made to this. The same program may be downloaded from a network to the apparatus, or a computer readable recording medium storing the same program may be installed in the apparatus. As the recording medium, any form may be used as long as a program can be stored and a computer can read. Specifically, as the recording medium, for example, an internal storage device mounted in the computer, such as a ROM or a RAM, a portable storage medium such as a CD-ROM, a flexible disk, a DVD disk, a magnetic optical disk or an IC card, a database storing computer programs, another computer and its database, a transmission medium on a line and the like can be enumerated. Besides, the function obtained by installation or download in advance may be realized in cooperation with an OS (Operating System) or the like of the apparatus.

The program may be an execution module in which a part or the whole is dynamically created.

Besides, it is needless to say that in various processes realized by causing the processor 801 to execute programs in the respective embodiments, at least a part thereof can be executed by a circuit in the ASIC 802.

As described above in detail, according to the technique described in the specification, the technique to detect the presence or absence of the curl of the sheet and the curl direction can be provided by the simple structure.

While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of invention. Indeed, the novel apparatus and methods described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the apparatus and methods described herein may be made without departing from the sprit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions. 

1. An image forming apparatus comprising: a sheet conveyance path to guide a sheet in a sheet conveyance direction; a first sensor; a first sensor lever which rotates around a first rotation shaft substantially parallel to a surface perpendicular to the sheet conveyance direction and substantially parallel to a surface of the sheet conveyed in the sheet conveyance path, and rotates between a standby position where the first sensor lever crosses the sheet conveyance path obliquely from the first rotation shaft to a sheet conveyance direction downstream side of the first rotation shaft and the first sensor is turned one of ON and OFF and a retract position where the first sensor lever is retracted from the sheet conveyance path and the first sensor is turned the other of ON and OFF; a first urging section to urge the first sensor lever from the retract position side to the standby position side; a second sensor; a second sensor lever which rotates around a second rotation shaft substantially parallel to the surface perpendicular to the sheet conveyance direction and substantially parallel to the surface of the sheet conveyed in the sheet conveyance path, and rotates between a standby position where the second sensor lever crosses the sheet conveyance path obliquely from the second rotation shaft to the sheet conveyance direction downstream side of the second rotation shaft and the second sensor is turned one of ON and OFF and a retract position where the second sensor lever is retracted from the sheet conveyance path and the second sensor is turned the other of ON and OFF; and a second urging section to urge the second sensor lever from the retract position side to the standby position side.
 2. The apparatus of claim 1, wherein the first sensor lever and the second sensor lever are arranged to be axisymmetric with each other with respect to a sheet conveying locus in the sheet conveyance direction.
 3. The apparatus of claim 1, wherein with respect to a gap of a sheet passing space of the sheet conveyance path in a direction perpendicular to the surface of the sheet conveyed in the sheet conveyance path, when viewed in the first rotation shaft direction, the gap in a range from a crossing position of a sheet contact surface of the first sensor lever in the sheet conveyance direction and a sheet contact surface of the second sensor lever to the first rotation shaft is wider than the gap at a position adjacent to a sheet conveyance direction upstream side of the first rotation shaft in the sheet conveyance direction.
 4. The apparatus of claim 3, wherein an expanding inner wall surface of the sheet conveyance path has a shape gradually expanding from the sheet conveyance direction upstream side and then gradually narrowing, and the sheet contact surfaces of the first and the second sensor levers at the standby position enter the sheet conveyance path from the inner wall surface of the gradually expanding portion.
 5. The apparatus of claim 3, wherein with respect to the sheet passing space in the sheet conveyance path, when viewed in the first rotation shaft direction, the gap at leading edge positions of the first and the second sensor levers at the standby position in the sheet conveyance direction is wider than the gap at the position adjacent to the sheet conveyance direction upstream side of the first rotation shaft in the sheet conveyance direction.
 6. The apparatus of claim 3, wherein with respect to the sheet passing space in the sheet conveyance path, when viewed in the first rotation shaft direction, the gap at sheet conveyance direction downstream side end positions of the sheet contact surfaces of the first and the second sensor levers at the retract position is wider than the gap at the position adjacent to the sheet conveyance direction upstream side of the first rotation shaft in the sheet conveyance direction.
 7. The apparatus of claim 1, wherein with respect to the sheet passing space in the sheet conveyance path, when viewed in the first rotation shaft direction, the gap at a position of the first rotation shaft is wider than the gap at the position adjacent to the sheet conveyance direction upstream side of the first rotation shaft in the sheet conveyance direction.
 8. The apparatus of claim 1, further comprising a curl determination section to determine that the sheet is curled to a side of one of the first and the second sensors, in which a change of a detection result from one of ON and OFF to the other is earlier and a change of a detection result from the other to the one is slower.
 9. The apparatus of claim 8, wherein the curl determination section determines that a curl amount of the conveyed sheet is large when a time difference between a change timing of a detection result from one of ON and OFF of the first sensor to the other and a change timing of a detection result from one of ON and OFF of the second sensor to the other is large.
 10. The apparatus of claim 1, wherein the first and the second sensor are optical sensors.
 11. A curl detecting method of an image forming apparatus including a sheet conveyance path to guide a sheet in a sheet conveyance direction, a first sensor, a first sensor lever which rotates around a first rotation shaft substantially parallel to a surface perpendicular to the sheet conveyance direction and substantially parallel to a surface of the sheet conveyed in the sheet conveyance path, and rotates between a standby position where the first sensor lever crosses the sheet conveyance path obliquely from the first rotation shaft to a sheet conveyance direction downstream side of the first rotation shaft and the first sensor is turned one of ON and OFF and a retract position where the first sensor lever is retracted from the sheet conveyance path and the first sensor is turned the other of ON and OFF, a first urging section to urge the first sensor lever from the retract position side to the standby position side, a second sensor, a second sensor lever which rotates around a second rotation shaft substantially parallel to the surface perpendicular to the sheet conveyance direction and substantially parallel to the surface of the sheet conveyed in the sheet conveyance path, and rotates between a standby position where the second sensor lever crosses the sheet conveyance path obliquely from the second rotation shaft to the sheet conveyance direction downstream side of the second rotation shaft and the second sensor is turned one of ON and OFF and a retract position where the second sensor lever is retracted from the sheet conveyance path and the second sensor is turned the other of ON and OFF, and a second urging section to urge the second sensor lever from the retract position side to the standby position side, the curl detecting method comprising determining a curl state of the sheet conveyed in the sheet conveyance path based on detection results of the first sensor and the second sensor.
 12. The method of claim 11, wherein the first sensor lever and the second sensor lever are arranged to be axisymmetric with each other with respect to a sheet conveying locus in the sheet conveyance direction.
 13. The method of claim 11, wherein with respect to a gap of a sheet passing space of the sheet conveyance path in a direction perpendicular to the surface of the sheet conveyed in the sheet conveyance path, when viewed in the first rotation shaft direction, the gap in a range from a crossing position of a sheet contact surface of the first sensor lever in the sheet conveyance direction and a sheet contact surface of the second sensor lever to the first rotation shaft is wider than the gap at a position adjacent to a sheet conveyance direction upstream side of the first rotation shaft in the sheet conveyance direction.
 14. The method of claim 13, wherein an expanding inner wall surface of the sheet conveyance path has a shape gradually expanding from the sheet conveyance direction upstream side and then gradually narrowing, and the sheet contact surfaces of the first and the second sensor levers at the standby position enter the sheet conveyance path from the inner wall surface of the gradually expanding portion.
 15. The method of claim 13, wherein with respect to the sheet passing space in the sheet conveyance path, when viewed in the first rotation shaft direction, the gap at leading edge positions of the first and the second sensor levers at the standby position in the sheet conveyance direction is wider than the gap at the position adjacent to the sheet conveyance direction upstream side of the first rotation shaft in the sheet conveyance direction.
 16. The method of claim 13, wherein with respect to the sheet passing space in the sheet conveyance path, when viewed in the first rotation shaft direction, the gap at sheet conveyance direction downstream side end positions of the sheet contact surfaces of the first and the second sensor levers at the retract position is wider than the gap at the position adjacent to the sheet conveyance direction upstream side of the first rotation shaft in the sheet conveyance direction.
 17. The method of claim 11, wherein with respect to the sheet passing space in the sheet conveyance path, when viewed in the first rotation shaft direction, the gap at a position of the first rotation shaft in the sheet conveyance direction is wider than the gap at the position adjacent to the sheet conveyance direction upstream side of the first rotation shaft in the sheet conveyance direction.
 18. The method of claim 11, further comprising determining that the sheet is curled to a side of one of the first and the second sensors, in which a change of a detection result from one of ON and OFF to the other is earlier and a change of a detection result from the other to the one is slower.
 19. The method of claim 11, further comprising determining that a curl amount of the conveyed sheet is large when a time difference between a change timing of a detection result from one of ON and OFF of the first sensor to the other and a change timing of a detection result from one of ON and OFF of the second sensor to the other is large.
 20. The method of claim 11, wherein the first and the second sensor are optical sensors. 